1. Field of the Invention
This invention relates generally to optical frequency conversion, and more particularly, it relates to arrangements for scanning an optical beam across an optical frequency converting medium to increase conversion efficiency.
2. Description of the Prior Art Including Prior Art Statement
Optical frequency conversion can be achieved by passing a light beam through an electro-optic crystal which develops light beam components at frequencies related to the frequency of the input beam in accordance with a parametric interaction process in the crystal. Maximum conversion of energy to the desired frequency occurs when the sum of the momentum vectors of all of the light waves participating in the parametric interaction process equals zero, a condition commonly referred to as the phase-match condition.
Optical frequency converting devices are finding increased use in a number of laser systems wherein there is a need to change the frequency of the originally generated laser beam. However, such devices are severely limited in their average power handling capabilities by thermal effects in the frequency-converting crystals. One undesirable thermal effect is that as the laser beam passes through the crystal, a portion of the laser energy is absorbed by the crystal to produce a temperature gradient across the crystal. Since the aforementioned phase-match condition is a function of the temperature of the crystal, temperature gradients across the crystal can make it impossible to obtain reasonably uniform phase-matching throughout the regions of the crystal traversed by the beam. Such mismatches can result in a significant reduction in the frequency conversion efficiency of the crystal as the average power level of the input beam increases.
In the past, the conversion efficiency of optical parametric crystals has been increased by passing the input light beam through cylindrical lenses to reshape the beam into a beam of elongated cross-section which impinges upon the crystal in an elongated illumination pattern. Such an illumination pattern reduces the temperature differential across the crystal, thereby achieving more uniform phase-matching across the crystal. Further details concerning the use of beam-shaping cylindrical lenses to increase the efficiency of optical frequency doublers are described in the paper "Effect of Laser and Medium Parameters on Second-Harmonic Generation," by V. D. Volosov, Soviet Physics--Technical Physics, Vol. 14, No. 12 (June 1970), pp. 1652-1658. Further discussions of the use of beam-shaping to reduce thermal gradients in optical frequency doubling crystals are found in the papers "High Average Power Frequency Doubling for Dye Laser Pumping," by D. T. Hon et al, Proceedings of the Society of Photo-Optical Instrumentation Engineers, Vol. 122 (Aug. 25-26, 1977), pp. 95-99; and "Average Power Breakthrough in Non-linear Process in Crystals," by D. T. Hon, IEEE Journal of Quantum Electronics, Vol. QE-13, No. 9 (September 1977), pp. 99D-100D.
As the average power level of the light beam applied to the parametric crystal is increased (by using higher average power lasers or pulsed lasers operating at higher pulse repetition frequencies, for example), it becomes difficult to achieve high conversion efficiency when lenses are employed to shape the beam illumination pattern on the crystal. This is due to the fact that as the beam cross-section is made more elongated to reduce thermal gradients in the crystal, the power density of the beam is reduced, thereby reducing conversion efficiency. The beam power density may be increased by using beams of higher peak power levels. Nevertheless, a limit is imposed on the ratio of beam average power to peak power for which high efficiency parametric frequency conversion can be achieved using the prior art.